I am getting confused, viscous fan, engine fan and electric fan? I am talking about the giant fan directly in front of the radiator, the one I have to take the blades off and remove the housing to work on things. I think this is the viscous fan/clutch.

My current guess, is that this is going. Rationale is its blowing weak, so if it blows weak cool air, things are stable, if its blowing weak hot air, then the coolant starts to overheat. Or the t-stat is restricting flow and again the warm air is just not sufficient with the weaker.

I'' get to work and report back with the solution.

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2003 X5 3.0i Silver (build 07/03)
2006 X5 3.0i Blue
1977 Ferrari 308 GTB
1994 Ferrari 348TS Speciale #14/15
1989 Ferrari Testarossa